WO2006120387A2

WO2006120387A2 - Sol-gel derived coating

Info

Publication number: WO2006120387A2
Application number: PCT/GB2006/001620
Authority: WO
Inventors: Heming Wang; Robert Akid
Original assignee: Sheffield Hallam University
Priority date: 2005-05-11
Filing date: 2006-05-03
Publication date: 2006-11-16
Also published as: GB0509606D0; WO2006120387A3; GB2425976A

Abstract

A sol-gel derived coating and method of preparing the same is provided. The coating is particularly suitable as an anticorrosion coating and may comprise additional functionality by the incorporation of one or more dopant compounds within the resulting ceramic coating. The sol-gel processing technique comprises formation of gel particulates suspended in a solution based slurry. The slurry is then applied to the substrate to form a first coat. Gellation of the particles is controlled to provide chemically active sites present within the first coat. A sol comprising an inorganic oxide is then applied to the first coat followed by subsequent heat treatment and/or drying to produce a thick, dense ceramic coating.

Description

SOL-GEL DERIVED COATING

Field of the Invention

The present invention relates to a sol-gel derived substrate coating and to a method of preparing a sol-gel derived coating.

Background to the Invention

Sol-gel processing has become a useful technique for producing ceramics of greater purity and homogeneity than conventional high temperature processes.

A sol is a dispersion of solid particles in a liquid phase, the particles being small enough to remain suspended indefinitely. A gel is a solid containing a liquid component in an internal network structure whereby both the liquid and solid are arranged in a highly dispersed state.

Sol-gel derived products are used across numerous fields however one of the more predominant applications is anticorrosion coatings associated with the automotive, aviation and construction industries.

US 6,284,682 discloses a method for preparing a sol-gel derived ceramic coating involving phosphating the sol-gel derived oxide or hydrated oxide and polymerising the phosphated product with heat treatment so as to decrease the porosity of the resulting ceramic. The sol-gel process may be used to fabricate dense, thick ceramic coatings for a variety of applications, including high temperature corrosion protection, wear resistance, bio-active ceramics and thermal barrier ceramics.

One disadvantage with producing ceramics by conventional sol-gel processing techniques is the resulting rough ceramic surface. Typically such ceramics require extensive polishing so as to meet surface gloss requirements. Manufacture of thick, dense ceramic coatings using sol-gel methods typically involves the dispersion of particulates within the sol. The greater the amount of particulates suspended in the sol, the more dense the resulting ceramic coating. A common problem therefore with conventional sol-gel processing techniques is the difficulty in stabilising particulate sol dispersions with the aim of producing a homogeneous ceramic coating.

Similarly, the manufacture of functional ceramics, such as biologically active ceramics, commonly involves dispersing one or more biological compounds or molecules in the sol prior to deposition of the sol onto the substrate. Using conventional sol-gel techniques it is difficult to configure and control the sol to prevent the biomolecules from reacting with other sol constituents and irreversibly destroying intended functionality of the bio-active ceramic. It is therefore difficult to both control dispersion of any suspended particles within the sol and at the same time prevent any biomolecules reacting with the sol constituents including any stabilising agents.

What is required therefore is a sol-gel derived coating and a method of preparing the same which solves the above problems.

Summary of the Invention

The inventors have devised a sol-gel process enabling the manufacture of thick, dense ceramic coatings. The coatings of the present invention are suitable as anticorrosion and antiwear coatings with additional functionality as disclosed herein by the incorporation of one or more dopant compounds or materials into the ceramic.

In order to produce a coating of significantly increased thickness over those obtained via conventional sol-gel techniques the inventors utilise a two stage process involving the preparation of a first layer formed from a slurry comprising a suspension of gel particulates, or alternatively termed gel precipitates. Following the deposition of the gel precipitates onto the substrate a second coating sol is applied directly over the gel precipitates such that the components of the second coating sol may readily diffuse into the layer formed by the gel precipitates so as to densify the coating.

According to specific implementations of the present invention a dopant compound or material may be incorporated within the second coating sol. One advantage of this is that the physical and chemical properties of the first sol used to generate the gel particulates may be optimised for example to ensure homogenous dispersion and the required amount of particle stabilisation. The resulting ceramic coating may be configured with the desired functionality by incorporating at least one dopant material or compound into the second coating sol. By forming the coating via this two stage process in which the gel precipitates are prepared separately from the sol containing the dopant material, it is possible to optimise and specifically tailor the physical and chemical properties of the second coating sol so as to provide an inert sol environment and/or to avoid destroying the functionality of any biologically active, bacterial or enzymic compounds dispersed within the second coating sol.

The enhanced coating thickness is achieved by firstly depositing a layer of gel particulates onto the substrate. The gel particulates are prepared as described herein using sol-gel techniques in which gellation is controlled and terminated at the desired point so as to generate gel nanoparticles. Due to incomplete gellation, the gel particles comprise chemically active sites, in particulate hydroxyl groups which may readily undergo condensation and other types of chemical reactions with the second coating sol subsequently and directly applied to the gel particulates located at the substrate.

Preferably, the substrate coated with the gel particulates, is dip coated or sprayed by the sol to produce a smooth coating surface finish of high surface gloss. -A-

By first coating the substrate with a layer of dried gel particulates followed by subsequent dip coating of this treated substrate with a sol comprising at least one inorganic oxide a thick, dense coating is produced obviating the need for conventional coating techniques used in the manufacture of conventional thick ceramic coatings.

According to a first aspect of the present invention there is provided a substrate having a coating, said coating comprising: a layer of metal oxide gel particulates, said particulates capable of reacting chemically with a sol comprising at least one inorganic oxide; and a sol applied to the particulate layer, said sol comprising at least one inorganic oxide and capable of diffusing into said layer and reacting chemically with said layer.

The sol may further comprise at least one dopant compound configured to effect the mechanical and/or chemical properties of the coating. The dopant compound may comprise any one or a selection of: nanoparticles; γ -alumina; at least one biologically active molecule; at least one species of bacteria; at least one type of enzyme; at least one dopant metal or metal ion; at least one dye compound; at least one metal carbide; or silicon carbide.

The sol is configured to undergo condensation and other chemical reactions with the gel particulates and may be a metal oxide sol, an alumina sol, an organic sol, or a silica sol.

Alternatively, the sol may comprise any one or a combination of the following:

ZrO₂; TiO₂; BeO; SrO; BaO; CoO; NiO; ZnO; PbO; CaO; MgO; CeO₂; Cr₂O₃; Fe₂O₃; Y₂O₃; Sc₂O₃; HfO₂; La₂O₃.

Preferably, the metal oxide gel particulates are dried alumina gel particulates. In particular, the gel particulates are nanoparticles formed from the incomplete gellation of a metal oxide or organic oxide sol using known sol-gel techniques. Altematively, the gel nano-particles may be formed from any one or a combination of the following: ZrO₂; TiO₂; BeO; SrO; BaO; CoO; NiO; ZnO; PbO; CaO; MgO; CeO₂; Cr₂O₃; Fe₂O₃; Y₂O₃; Sc₂O₃; HfO₂; La₂O₃.

The coating and the coating process of the present invention is suitable for use with a variety of different substrate materials including metal, in particular aluminum, aluminum alloy; aluminum alloy 2024-T3; stainless steel; glass or ceramic; carbon steel; galvanised steel; electroplated carbon steel; chrome/nickel electroplated steel; or any ferrous based alloy.

According to a second aspect of the present invention there is provided a method of preparing a sol-gel derived coating, said method comprising: preparing a slurry of metal oxide gel particulates; coating a substrate with said slurry; and drying said slurry to produce a dry gel coating, said coating comprising sites capable of reacting chemically with a sol comprising at least one inorganic oxide.

Preferably, the method further comprises preparing a sol comprising at least one inorganic oxide; applying said sol to said gel coating whereby said gel coating is capable of reacting chemically with said sol; and heat treating said sol and said gel coating to form a ceramic coating.

Preferably, the method further comprises mixing at least one dopant compound with said sol prior to said step of applying said sol to said gel coating.

Preferably, the method further comprises stabilising said slurry of said metal oxide gel particulates using mechanical means.

Preferably, the method further comprises stabilising said slurry of said metal oxide gel particulates by adding a polymer compound to said slurry. The stabilising polymer may comprise polyvinylalcohol (PVA), polypropylene glycol, piperazine, triethylenetriamine (TETA), diethylenetriamine (DEA), tetraethylenepentamine (TEPA) or mixtures thereof. Preferably, the step of drying the slurry comprises heat treating the slurry.

Preferably, the step of preparing the slurry of gel particulates comprises: preparing a metal oxide sol; mixing said metal oxide sol with a first organic solvent to generate metal oxide gel particulates; filtering said gel particulates; drying said gel particulates; mixing said gel particulates with a second organic solvent.

The viscosity of the slurry may be adjusted by adjusting a concentration of the first and/or the second organic solvent.

Preferably, where the gel particulates are γ -alumina the step of drying the slurry involves heat treating the gel particulates at 350⁰C and the step of heat treating the sol and the gel coating comprises heating the coating at a temperature of at least 100⁰C. Additionally, the step of drying the slurry may involve heating the slurry at a temperature of at least 50°C. Alternatively, the step of drying the slurry may involve drying at room temperature.

According to a third aspect of the present invention there is provided a method of preparing a sol-gel derived coating, said method comprising: preparing metal oxide particulate aggregates from a sol-gel process; mixing said particulate aggregates with an organic solvent to form a slurry; chemically or mechanically stabilising dispersion of said particulate aggregates in said slurry; applying said slurry to a substrate; drying said slurry to form said coating.

Preferably, the method further comprises: preparing a sol comprising at least one inorganic oxide; applying said sol to said coating; allowing said sol to diffuse into said coating; drying said sol and said coating to form a ceramic coating.

Additionally, the inventors have realised that the porosity of the ceramic coating according to the present invention may be controlled by heat treating or drying the gel particles and/or the resulting coating formed from the gel particles and the sol comprising at least one inorganic oxide.

In particularly the coating of the present invention, comprising at least one dopant material, may be configured as a bio-active material; or as a bio-sensor or as a catalytic coating, for example incorporating metal particles or metal ions for the growing of nanotubes and the like.

The present invention is also suitable for use with a pre-treated substrate on which at least one layer or coating is already present. Alternatively or in addition, the substrate may be pre-treated, particularly where the substrate is a metal or metal alloy, to remove any oxidised layer which may otherwise inhibit binding of the ceramic coating. Accordingly, the substrate may be etched using known acid/alkali treatments.

Using the sol-gel coating method of the present invention it is easy to control the composition of the coating and obtain a dense, thick coating which exhibits reduced cracking susceptibly. Moreover, the concentration of the components of the sols of the present invention may be very high which is important to generate a thick coating via a single dip coating step.

Brief Description of the Drawings

For a better understanding of the invention and to show how the same may be carried into effect, there will now be described by way of example only, specific embodiments, methods and processes according to the present invention with reference to the accompanying drawings in which:

Fig 1 herein illustrates schematically formation of the sol-gel derived coating according to one aspect of the present invention.

Detailed Description There will now be described by way of example a specific mode contemplated by the inventors. In the following description numerous specific details are set forth in order to provide a thorough understanding. It will be apparent however, to one skilled in the art, that the present invention may be practiced without limitation to these specific details. In other instances, well known methods and structures have not been described in detail so as not to unnecessarily obscure the description.

To prepare the thick, dense anticorrosion coating of the present invention a first metal oxide sol is prepared 100 involving a mixed aqueous-organic solution using conventional metal-organic pre-cursors. Subsequent gellation of the first sol is controlled to obtain gel particulates 101 , alternatively termed gel precipitates, formed in the mixed water-organic solvent.

The gel particulates are then filtered using conventional filter means at step

102. The gel particulates are then suspended in a solvent to form a particulate slurry 103 with continued stirring. The substrate is then coated with the particulate slurry 104 and the coating dried 105 to form a first layer of dried gel particulates.

A second layer sol is prepared, the sol comprising at least one inorganic oxide prepared from metal-organic pre-cursors using conventional sol-gel methods.

To provide the resulting ceramic coating with the desired functionality, a dopant compound or material is dispersed in the sol 107. The substrate comprising the first layer of gel particulates is then coated with the second layer sol 108. It is considered that curing of the resulting coating occurs via diffusion of the second layer sol into the first layer coat formed by the gel particulates. The coating is finally dried 109 to produce a homogeneous ceramic coating.

The present invention is particularly adapted to produce thick, dense ceramic coatings. This is made possible by preparing a gel particulate slurry of high gel particulatθ concentration initially separated from a second sol comprising the curing agent. Conventional sol gel methods for preparing anticorrosion coatings via one pot pathways, in which the curing agent is mixed with the metal oxide sol prior to deposition onto the substrate, are significantly limited as to the thickness 5 and density of the coating obtainable.

Additionally, by controlling gellation of the metal oxide sol, gel particulates are prepared comprising a plurality of hydroxyl groups which when the particulates are dried provide chemically active sites or reactive sites. This allows the gel particles o to chemically bond with other molecules or compounds and in particular selected constituents of the second layer sol.

To improve the homogeneity - of the resulting ceramic, a stabilising agent is added to the gel particulate slurry. In particular, the inventors have observed 5 increased homogeneity by sequential addition of amounts of stabilising agent over time with stirring. In this way it is possible to stabilise the particulate gel suspension for a considerable length of time, of the order of months.

The thickness of the resulting ceramic coating may be further increased by o incorporating γ -alumina or similar nanoparticles within the second layer sol 107.

Depending upon the constituents of the particulate slurry and the doped second sol, and the desired final coating, it may be appropriate to heat treat the first layer formed from the gel particulates. In particular, the porosity of the first 5 particulate coating is controlled at stage 105 by the temperature at which the coating is dried and the drying time.

According to further specific implementations of the present invention, the gel particulate coating may not be dried following coating of the substrate at stage 104 o and prior to deposition of the second coat sol onto the substrate. Alternatively, the porosity of the gel particulate layer may be affected by treating the layer with known chemicals or products configured to decrease porosity.

Additionally, prior to the final heat treatment or drying step of the coating formed by the first coat slurry and the second coat sol, the final coating can be modified by treatment with other, neutral, acid or alkali solutions such as Na₃PO₄, H₃PO₄ or NaOH.

Example 1

The Preparation of the First-Coat Slurry and Substrate Treatment

10Og aluminium isopropoxide was added to a solution of 200 ml distilled water, 50ml propanol alcohol, and 1 ml concentrated nitric acid under vigorous stirring at a temperature above 8O⁰C. 5 ml concentrated nitric acid was added to the solution to obtain a clear alumina sol.

The prepared alumina sol was mixed dropwise with NH₃-H₂O solution to produce the gel particulates, alternatively termed alumina gel precipitates, in the mixed water-organic solvent. The NH₃-H₂O solution was vigorously stirred during processing.

The above solution was filtered to obtain precipitated gel nano-particles, which were then dried at room temperature.

The gel nano-particles were added to an organic solvent of 150 ml 2- propanol and 50 ml methanol under vigorous stirring. 10 ml acetic acid and 20 ml triethylenetriamine were added to stabilise the solution.

The solution was stirred continuously until a slurry was obtained. The viscosity of the slurry may be adjusted by the concentration of the organic solvents used.

Aluminium alloys (such as AA2024-T3), glasses, ceramics or stainless steels were used as the substrates which were dip-coated into the slurry. The coat was then dried at room temperature or a higher temperature (e.g. 80⁰C) to obtain the first coat without cracking of the coating.

The Preparation of the Second-Coat Sol and Substrate Treatment

10Og aluminum isopropoxide was added to a solution of 200ml distilled water, 50ml propanol alcohol, and 1ml concentrated nitric acid under vigorous stirring in the temperature above 8O⁰C. Another 5ml concentrated nitric acid was added in the solution to get the clear alumina sol.

By way of example, 1.0% nanoparticles of silicon carbide may be added to this solution if required.

The dried first-coat sample was dip coated into this solution and then dried at room temperature or a higher temperature (e.g. 80⁰C).

The sample was finally heated at the required temperature, such as 200°C, 400°C, 1000⁰C or other temperatures; the temperature of heat treatment being dependent on the substrate material, and/or the expected or desired properties of the coating.

Claims

Claims:

1. A substrate having a coating, said coating comprising:

a layer of metal oxide gel particulates, said particulates capable of reacting chemically with a sol comprising at least one inorganic oxide; and

a sol applied to said layer of particulates, said sol comprising at least one inorganic oxide and capable of diffusing into said layer and reacting chemically with said layer.

2. The substrate as claimed in claim 1 wherein said sol further comprises at least one dopant compound configured to effect the mechanical and/or chemical and/or optical properties of said coating.

3. The substrate as claimed in claim 2 wherein said dopant compound is formed as nanoparticles.

4. The substrate as claimed in claim 2 wherein said dopant compound is γ -alumina.

5. The substrate as claimed in claim 2 wherein said dopant compound is at least one biologically active molecule.

6. The substrate as claimed in claim 2 wherein said dopant compound is at least one species of bacteria.

7. The substrate as claimed in claim 2 wherein said dopant compound is at least one dopant metal or metal ion.

8. The substrate as claimed in claim 2 wherein said dopant compound is at least one dye compound.

9. The substrate as claimed in claim 2 wherein said dopant compound is silicon carbide or a metal carbide.

10. The substrate as claimed in any preceding claim wherein said sol is 5 any one or a combination of the following set of:

• an alumina sol;

• a silica sol.

o 11. The substrate as claimed in any preceding claim wherein said sol comprises any one or a combination of the following set of:

ZrO₂; TiO₂; BeO; SrO; BaO; CoO; NiO; ZnO; PbO; CaO; MgO; CeO₂; Cr₂O₃; Fe₂O₃; Y₂O₃; Sc₂O₃; HfO₂; La₂O₃. 5

12. The substrate as claimed in any preceding claim wherein metal oxide gel particulates are nanoparticles.

13. The substrate as claimed in any preceding claim wherein said metal o oxide gel particulates are alumina gel particulates.

14. The substrate as claimed in any preceding claim wherein said substrate is an aluminium alloy.

5 15. The substrate as claimed in any one of claims 1 to 13 wherein said substrate is a glass.

16. The substrate as claimed in any one of claims 1 to 13 wherein said substrate is a ceramic. 0

17. The substrate as claimed in any one of claims 1 to 13 wherein said substrate is stainless steel.

18. The substrate as claimed in any one of claims 1 to 13 wherein said substrate is carbon steel.

19. The substrate as claimed in any one of claims 1 to 13 wherein said 5 substrate is galvanised steel.

20. The substrate as claimed in any one of claims 1 to 13 wherein said substrate is chrome/nickel electroplated steel.

o

21. A method of preparing a sol-gel derived coating, said method comprising:

preparing a slurry of metal oxide gel particulates;

5 coating a substrate with said slurry; and

drying said slurry to produce a dry gel coating, said coating comprising sites capable of reacting chemically with a sol comprising at least one inorganic oxide.

o

22. The method as claimed in claim 21 further comprising:

preparing a sol comprising at least one inorganic oxide;

applying said sol to said gel coating whereby said gel coating is capable of 5 reacting chemically with said sol; and

heat treating said sol and said gel coating to form a ceramic coating.

23. The method as claimed in claim 22 further comprising: 0 mixing at least one dopant compound with said sol prior to said step of applying said sol to said gel coating.

24. The method as claimed in claim 23 wherein said dopant compound is formed as nanoparticles.

5 25. The method as claimed in claims 23 or 24 wherein said dopant compound is alumina.

26. The method as claimed in claims 23 or 24 wherein said dopant compound γ-alumina. 0

27. The method as claimed in claim 23 wherein said dopant compound is at least one biologically active molecule.

28. The method as claimed in claim 23 wherein said dopant compound 5 is at least one species of bacteria.

29. The method as claimed in claim 23 wherein said dopant compound is at least one dopant metal or metal ion.

0 30. The method as claimed in claim 23 wherein said dopant compound is at least one dye compound.

31. The method as claimed in claim 23 wherein said dopant compound is silicon carbide or metal carbide. 5

32. The method as claimed in any one of claims 21 to 31 further comprising:

stabilising said slurry of said metal oxide gel particulates using mechanical o means.

33. The method as claimed in any one of claims 21 to 31 further comprising:

stabilising said slurry of said metal oxide gel particulates by adding a polymer 5 or organic compound to said slurry.

34. The method as claimed in claim 33 wherein said polymer compound is polyvinylalcohol and/or triethylenetriamine.

0 35. The method as claimed in any one of claims 21 to 34 wherein said step of drying said slurry comprises heat treating said slurry.

36. The method as claimed in any one of claims 21 to 35 wherein said metal oxide gel particulates are alumina gel particulates. 5

37. The method as claimed in any one of claims 21 to 35 wherein said sol comprises any one or a combination of the following set of:

SiO₂; AI₂O₃; ZrO₂; TiO₂; BeO; SrO; BaO; CoO; NiO; ZnO; PbO; CaO; MgO; o CeO₂; Cr₂O₃; Fe₂O₃; Y₂O₃; Sc₂O₃; HfO₂; La₂O₃.

38. The method as claimed in any one of claims 21 to 37 wherein said step of preparing said slurry of gel particulates comprises:

5 preparing a metal oxide sol;

mixing said metal oxide sol with a first organic solvent to generate metal oxide gel particulates;

o filtering said gel particulates;

drying said gel particulates; mixing said gel particulates with a second organic solvent.

39. The method as claimed in claim 38 wherein a viscosity of said slurry 5 may be adjusted by adjusting a concentration of said first and/or said second organic solvent.

40. The method as claimed in any one of claims 21 to 39 wherein said step of heat treating said sol and said gel coating comprises heating at a o temperature of at least 100⁰C.

41. The method as claimed in any one of claims 21 to 40 wherein said step of drying said slurry comprises drying said slurry at room temperature.

5 42. The method as claimed in any one of claims 21 to 40 wherein said step of drying said slurry comprises heating said slurry at a temperature of at least 5O⁰C.

43. A method of preparing a sol-gel derived coating, said method 0 comprising:

preparing metal oxide particulate aggregates from a sol-gel process;

mixing said particulate aggregates with an organic solvent to form a slurry; 5 chemically or mechanically stabilising dispersion of said particulate aggregates in said slurry;

applying said slurry to a substrate; 0 drying said slurry to form said coating.

44. The method as claimed in claim 43 further comprising:

preparing a sol comprising at least one inorganic oxide;

applying said sol to said coating;

allowing said sol to diffuse into said coating;

drying said sol and said coating to form a ceramic coating.

45. The method as claimed in claims 43 or 44 wherein said metal oxide particulate aggregates comprise any one or a combination of the following set of:

AI₂O₃; ZrO₂; TiO₂; BeO; SrO; BaO; CoO; NiO; ZnO; PbO; CaO; MgO; CeO₂; Cr₂O₃; Fe₂O₃; Y₂O₃; Sc₂O₃; HfO₂; La₂O₃.

46. The method as claimed in any one of claims 43 to 45 wherein said sol comprises any one or a combination of the following set of:

SiO₂; AI₂O₃; ZrO₂; TiO₂; BeO; SrO; BaO; CoO; NiO; ZnO; PbO; CaO; MgO;

CeO₂; Cr₂O₃; Fe₂O₃; Y₂O₃; Sc₂O₃; HfO₂; La₂O₃.

47. The method as claimed in any one of claims 43 to 46 further comprising adding a dopant compound to said sol prior to said step of applying said sol to said coating, said dopant compound comprising any one or a combination of the following set of:

• nanoparticles;

• γ -alumina; • a biologically active molecule;

• a species of bacteria;

• a dopant metal or metal ion; • a dye compound.